Cellular-type mobile communication has evolved from UMTS (Universal Mobile Telecommunication System) to LTE (Long Term Evolution). In LTE, an OFDM (Orthogonal Frequency Division Multiplexing) based system is provided as a wireless access technique. According to LTE, high-speed wireless packet communication with a downlink peak transmission rate of 100 Mbps or more and an uplink peak transmission rate of 50 Mbps or more becomes possible.
Currently, the 3GPP (3rd Generation Partnership Project), an international standardization body, has started examination of LTE-A (LTE-Advanced), an LTE-based mobile communication system, for realization of higher-speed communication. LTE-A aims at a downlink peak transmission rate of 1 Gbps and an uplink peak transmission rate of 500 Mbps, and various new techniques, such as wireless access methods and network architectures, are examined (for example, Non-Patent document 1). On the other hand, LTE-A is required to be compatible with LTE because it is an LTE-based system.
As one of methods for performing high-speed data communication, a method of introducing a relay station to support communication between a base station and a mobile station is examined. The relay station intervenes between a conventional base station and mobile station, and it is installed to support high-speed data communication. As the relay station, for example, a relay station which only amplifies a wireless signal (a data signal and noise) (a repeater system), a relay station capable of amplifying only a data signal in a wireless signal (a decode-and-forward system), a relay station implemented with functions of Layer 2 (L2: (such as a MAC (Media Access Control) layer or the like) (an L2 relay station), and a relay station implemented with functions of Layer 3 (L3 (an RRC (Radio Resource Control) layer)) and behaving as a station having functions equivalent to those of a base station (L3 system) are examined.
A method of developing relay stations in a cell is also examined. For example, a method of developing relay stations at cell ends for the purpose of increasing the throughput at the cell ends or a method of developing relay stations within a range where radio waves do not reach (blind zones) are examined.    [Non-Patent document 1] 3GPP TR 36.913, “Requirements for further advancements for Evolved UniversaLTErrestrial Radio Access (E-UTRA) (LTE-Advanced)”, V8.0.1, Release 8, May 2009.
In data communication via a relay station, the relay station is involved in conventional data communication between a base station and a mobile station. As matters to be examined for scheduling of data transmission in consideration of a relay station, a wireless resource management method and an HARQ (Hybrid Automatic Repeat Request) control method are given. Here, HARQ is a data retransmission system in which a retransmission pattern is determined in consideration of the point that, on the receiving side, such data that decoding has failed may be combined with retransmitted data without being discarded.
From a viewpoint of a scheduling execution place, scheduling is roughly classified into two systems: centralized scheduling and distributed scheduling. In the centralized scheduling, a base station which controls a relay station executes scheduling of data transmission related to relay stations under the base station and mobile stations under the relay stations. On the other hand, in the distributed scheduling, a base station which controls relay stations executes only scheduling of data transmission related to mobile stations connected to the base station itself, and the relay stations execute only scheduling of data transmission related to mobile stations related to the relay stations themselves.
As for the wireless resource management method, in LTE, management of wireless resources is controlled by the RRC layer of a base station. In comparison, in LTE-A, a relay station is also involved in communication. Therefore, a wireless resource management place and method are examined. In LTE-A, one or more L3-system relay stations having functions equivalent to those of a base station may be installed in the cell of a base station. Here, the base station controlling the relay stations are called a donor base station (Donor eNB). The donor base station and the relay stations may communicate between the RRC layers. Therefore, by performing wireless resource management in cooperation between the RRC layers, it is possible to perform efficient wireless resource management.
As the method for wireless resource management by scheduling, there are dynamic scheduling and semi-persistent scheduling (SPS). The dynamic scheduling is used for non-real-time type communication such as communication for web browsing. In the dynamic scheduling, wireless resources used for a PDCCH (Physical Downlink Control Channel) are specified in both of uplink communication and downlink communication. In comparison, the semi-persistent scheduling (SPS) is used for real-time type communication represented by VoIP (Voice over IP). In the SPS, wireless resources are fixedly allocated for a certain predetermined period, before communication is actually executed. For example, in VoIP communication, initial transmission of data occurs every 20 ms. Therefore, wireless resources to be used are notified every 20 ms. However, when retransmission of data is executed, the dynamic scheduling is used.
As the HARQ control method, there is a control system described below. In LTE, an asynchronous HARQ system is adopted for downlink communication, and a synchronous HARQ system is adopted for uplink communication.
In LTE-A, it is required to support the above asynchronous HARQ system and synchronous HARQ system for at least LTE mobile stations in order to secure compatibility with LTE.
The asynchronous HARQ system is a method of receiving a confirmation response (ACK or NACK) to downlink transmission after 4 ms and, when the confirmation response is NACK, executing downlink retransmission at an arbitrary timing. In the asynchronous HARQ system, a base station is required to notify a mobile station of data transmission, necessarily using a PDCCH. On the other hand, the asynchronous HARQ system is a method of receiving a confirmation response to uplink transmission after 4 ms and, when the confirmation response is NACK, executing uplink retransmission 4 ms after the reception. In the asynchronous HARQ system, the base station is not required to notify a mobile station of data transmission by a PDCCH. In other words, a mobile station may retransmit data without receiving notification by the PDCCH. As described above, in the asynchronous HARQ system, it is possible to retransmit data without a PDCCH and, therefore, reduce signaling overhead.
When a relay station is involved in data communication between a base station and a mobile station in LTE-A, data processing time at the relay station occurs. Therefore, development of a method for maintaining the timing provided for the synchronous HARQ system or the asynchronous HARQ system as described above has been desired. That is, there may be a case that compatibility with the synchronous HARQ system or asynchronous HARQ system provided in LTE cannot be maintained due to intervention of a relay station between a mobile station and the base station.
FIG. 20 is a diagram illustrating an example of HARQ timing in uplink communication. FIG. 20 illustrates an example of a case that data is transmitted from a mobile station (UE: User Equipment) to a donor base station (DeNB) via a relay station (RN: Relay Node).
In the example illustrated in FIG. 20, wireless resources (a transmission timing) used for data transmission by the mobile station (UE) and the relay station (RN) are determined in advance by the SPS. In the example illustrated in FIG. 20, the UE transmits data (data 1) to the relay station at a timing in accordance with the SPS, that is, at a sub-frame number “0” (1 sub-frame: 1 ms) between the mobile station and the relay station.
The relay station relays the data 1 from the mobile station to the donor base station at a timing in accordance with the SPS, that is, at a sub-frame number “8” between the relay station and the donor base station. The donor base station returns a confirmation response (ACK or NACK) to the relay station at a sub-frame number “12” 4 ms after the sub-frame number “8” in accordance with the synchronous HARQ system. At this time, when failing in decoding (normal reception) of the data 1, the donor base station returns a NACK (HARQ NACK) message indicating the failure, to the relay station at the sub-frame number “12” in accordance with the synchronous HARQ system.
The relay station transfers the HARQ NACK message to the mobile station at a sub-frame number “12” between the mobile station and the relay station corresponding to 4 ms after the sub-frame number “12” in accordance with the synchronous HARQ system. Therefore, the mobile station receives the HARQ NACK message to the data 1 at the sub-frame number “12.” Then, the mobile station retransmits the data 1 at a sub-frame number “16” after 4 ms in accordance with the synchronous HARQ system.
According to the synchronous HARQ system in LTE, however, the HARQ NACK message (HARQ feedback) from the donor base station should be received at a sub-frame number “4” 4 ms after the sub-frame number “0.” The retransmission of the data 1 from the mobile station should be performed at a sub-frame number “8” after elapse of 4 ms after the sub-frame number “4.” As described above, there is a problem that, when the relay station intervenes, an operation which is not compatible with the synchronous HARQ system of LTE occurs.
FIG. 21 is a diagram illustrating an example of HARQ timing in downlink communication. FIG. 21 illustrates an example of a case that data is transmitted from a donor base station (DeNB) to a mobile station (UE: User Equipment) via a relay station (RN: Relay Node). In the example illustrated in FIG. 21, the donor base station transmits data to a mobile station every ten sub-frames in accordance with the SPS. For example, data (data 1) from the donor base station is transmitted to the relay station at a sub-frame number “0” between the relay station and the base station. The relay station transfers the data 1 to the mobile station at a sub-frame number “4” (a sub-frame number “0” between the mobile station and the relay station) after elapse of 4 ms.
The mobile station returns a confirmation response (ACK or NACK) to the relay station at a sub-frame number “4” 4 ms after the sub-frame number “0” in accordance with the asynchronous HARQ system. In this case, when decoding (normal reception) of the data 1 fails, a NACK (HARQ NACK) message is returned.
The relay station returns a NACK message to the donor station at a sub-frame number “12” (between the relay station and the donor station) after elapse of 4 ms in accordance with the asynchronous HARQ system. The donor base station retransmits the data 1 to the relay station at an arbitrary timing, for example, at a sub-frame number “16” as illustrated in FIG. 21 in accordance with the asynchronous HARQ system. The relay station relays the retransmission of the data 1 to the mobile station after elapse of 4 ms.
As described above, when the relay station intervenes, the donor station cannot receive a confirmation response at a timing in accordance with the asynchronous HARQ system (the original reception timing is the sub-frame number “4”) even in downlink communication.